Manipulable powered apparatus termed "tree falling heads" are used in tree falling operations. Conventional tree falling heads include grapple arms for holding the tree while it is cut and a power saw for cutting the tree. The present application is particularly concerned with tree falling heads having circular power saws which can be designed to provide a platform upon which the tree is supported while it is cut and upon which the tree may be transported after cutting. Such an arrangement is shown in U.S. Pat. No. 3,915,209 issued Oct. 28, 1975 for an invention of Bernard Denis, in which a cutter-bearing ring is drivingly rotated around the periphery of a support platform. Denis provides a drive gear which is offset to one side of the circular platform/ring assembly and which engages gear teeth provided around the inner circumference of the cutter-bearing ring. A cutter ring suffers a number of disadvantages as compared with a cutter disc. For example, the ring must be precision machined to minimize wear on the drive gear. In practice, because it is difficult to provide adequate support for a cutter ring, the loads to which the ring is subjected affect the ring/drive gear tolerance and cause premature wearing of the drive gear. It is also difficult to lubricate a cutter ring adequately, which further accelerates wear.
U.S. Pat. No. 3,494,389 issued Feb. 10, 1970 for an invention of John L. Thibodeau illustrates a ring saw which is rotated by a drive gear offset to one side of the ring to mesh with gear teeth disposed around the inner circumference of the ring. The offset drive mechanism leaves the ring saw unobstructed over most of its diameter, thereby increasing the size of tree which may be cut with a saw of a given diameter. Unlike Denis, Thibodeau provides no support platform--presumably because his apparatus does not appear to be intended for use in cutting trees, but rather is used for slashing and loading small bundles of timber which apparently do not have to be supported away from the rotating saw while they are cut, as does a tree which would pinch and stall the saw if allowed to rest on the saw during the cutting operation. Since Thibodeau provides no platform to support the tree, his apparatus is unsuited to use in tree falling operations.
A particular problem with prior art power saws concerns the saw cutting elements which must be kept sharp in order to produce an even, clean cut. If the saw cutting elements are dull, they tend to leave a ragged edge and may cause splintering at the severed butt of the tree, seriously degrading the quality of the lumber subsequently produced from the tree and reducing its value, particularly in foreign markets.
Saws having fixed saw teeth of the type shown in the Thibodeau patent are particularly difficult to keep sharp. An experienced saw filer must be engaged to sharpen the saw teeth at regular intervals. If any of the saw teeth are broken or damaged to the point that they cannot be sharpened (a common occurrence, since tree falling heads are operated close to the ground by an operator who may be more than twenty feet away from the saw, thereby significantly increasing the likelihood that the saw may strike a rock or other object and damage the saw teeth), then the remains of the damaged teeth must be removed and new teeth silver-soldered in their place. Thus, Thibodeau's arrangement is undesirable because of the costly labor-intensive process required to sharpen and/or repair the saw teeth. Furthermore, unless an expensive inventory of sharpened saws is maintained, significant down time may be experienced in view of the time required to disassemble the saw and transport it from a remote site of operation to a facility having apparatus and personnel capable of sharpening and/or repairing the saw (or conversely, down time may be experienced in view of the time and cost inherent in transporting such apparatus and personnel to the remote operation site). Although Denis overcomes some of the disadvantages of Thibodeau's arrangement by providing individually replacable cutting teeth with a rather elaborate mechanism for locking each tooth in place around the rim of the saw, his arrangement is undesirable in view of its relative complexity, which increases the overall cost of the saw.
To overcome the foregoing disadvantages, the present invention provides disposable, replacable, self-sharpening cutting elements which are cheap to make and which may be rapidly installed by inexperienced personnel. The teeth are removably attached around the periphery of the cutting disc by holders which have a relatively simple design yet provide massive support capable of withstanding and transmitting to the cutting disc forces imparted to the cutting elements during cutting operations.